Test heads are often utilized in the testing of integrated circuits. In order to use a test head to test integrated circuits, the test head is typically “docked” to a piece of peripheral equipment such as a prober or a device handler (hereinafter “peripheral”). A test head manipulator is typically used to position and manipulate the test head during the docking operation.
In docking a test head to a peripheral, it is desirable that the test head be moveable in a number of directions (i.e., that the test head have a number of degrees of freedom). Further, it is, also desirable to have the test head be compliantly moveable with respect to various degrees of freedom (i.e., the test head is substantially weightless or may be moved with a relatively small amount of externally applied force with respect to each of the degrees of freedom).
As a test head is moved through space, both its displacement and angular orientation with respect to the manipulator changes. For the purposes of describing the present invention, it is convenient to define two rectangular coordinate systems, one coordinate system attached to the test head and one coordinate system attached to the manipulator.
FIG. 1 depicts test head 150 having mutually orthogonal coordinate axes I 102, J 104, and K 106 attached thereto. Thus, this set of axes moves through space with the test head. Test site 160 is located on a surface of test head 150 and it includes electrical contacts which are to be placed in contact with the device under test (“dut”). The J axis 104 is shown orthogonal to and directed in a positive direction away from the test site surface. Rotation about I 102 is generally referred to as “pitch,” “tumble,” or “nod.” Rotation about K 106 is generally referred to as “roll.” Rotation about J 104 is generally referred to in the industry as “theta.” Briefly, when the test head is docked, peripheral test site 160 is desirably aligned with a corresponding site on the peripheral. Generally, with test head 150 slightly away from the peripheral, test head 150 is first aligned in five degrees of freedom including I, K displacements, pitch and roll rotations (establishes planarity) and theta rotation. Test head 150 is then maneuvered in the J direction until the test contacts are properly engaged; whereupon it is docked. Often, precise alignment features are provided to achieve a precise final alignment as test head 150 is maneuvered into a final docked position. Thus, it is generally desirable to enable the test head to be maneuvered simultaneously in six degrees of freedom for docking.
FIG. 2 illustrates a set of mutually orthogonal coordinate axes 200 useful for describing motions of the manipulator. This set of coordinate axes is fixed to the manipulator and thus fixed in space. X-axis 202 illustrates a linear side-to-side direction. Likewise, Y-axis 204 illustrates a linear up-down or vertical direction along which a test head is desirably moveable, and Z-axis 206 illustrates a linear in-out direction along which the test head is desirably moveable. Rotations about these three axes are referred to, for purposes of this application, as “U,” “V,” and “W” respectively.
If a test head can move (in conjunction with the test head manipulator) along and rotate about each of X-axis 202, Y-axis 204, and Z-axis 206, the manipulator is said to provide at least six (6) degrees of freedom. If a test head can be moved compliantly, both linearly and rotationally, with respect to its axes I 102, J 104, and K 106 then the test head is said to be compliant with six (6) degrees of freedom.
Because test heads are typically very expensive, it is often desirable to use the same test head to dock with various different peripherals. For example, the same test head may be used to dock in a horizontal plane (i.e., with J 104 vertical) with a device handler (e.g., a test head may dock with a device handler from below the device handler) and a prober (e.g., a test head may dock with a prober from above the prober). In order to dock with various different types of peripherals, a test head manipulator desirably has a long vertical stroke (e.g., a long vertical range of motion). However, because of size constraints on test heads and the associated manipulators, this is not always practical. Additionally, certain test head manipulator systems utilize pneumatic cylinders to position and manipulate test heads in the vertical direction. In such a design, the vertical stroke provided by the test head manipulator is limited by the stroke of the pneumatic cylinder arrangement. Often, with larger test heads, the stroke of a pneumatic cylinder arrangement is inadequate to provide a vertical range of motion adequate for docking a test head with the different types of peripherals.
In other instances it may be required to dock with a peripheral where test site 160 is in a vertical plane. In such a case, test head 150 must be rolled 90 degrees from the horizontal so that J 104 axis is in a horizontal plane. Then, theta rotation occurs in a vertical plane. With respect to the fixed manipulator axes 200, it is common for such vertical plane docking to be required in either the XY plane or the YZ plane. Other vertical planes and arbitrarily angled planes are of course also possible.
Another problem that arises when trying to dock a test head with different types of peripherals is that during manipulation of the test head, a degree of freedom may be lost. For example, in certain configurations where a test head is in a position where its J 104 axis is parallel to the X-axis of the manipulator (i.e., when docking in the YZ plane), the theta degree of compliant freedom may be lost.
Conventional attempts at alleviating such a lost degree of freedom have resulted in increasingly complex and expensive manipulator systems. For example, Holt at U.S. Pat. No. 5,450,766 and Slocum at U.S. Pat. No. 5,931,048 show apparatuses which avoid the aforementioned situation.
As provided above, in systems for the docking of a test head, it is sometimes desirable to provide complaint motion in each of the test head's six (6) degrees of freedom. This means that during docking, a test head manipulator desirably balances the test head in a substantially weightless condition in each of the these six (6) degrees of freedom such that an operator can move the test head manually in each of the directions with relatively little force. However, as test heads have become larger and heavier, the physical force required to manually manipulate the test head in certain directions (even in a compliant state) may be difficult if not impossible for certain operators to provide.
As such, it would be desirable to provide a test head positioning and manipulation system addressing the above recited deficiencies.